Explain temperature effects on rate constants
Explain qualitatively why raising temperature increases the rate constant and usually the reaction rate.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 26. Rate laws are empirical unless an elementary step is explicitly stated. Written practice is self-checked
Before you begin
- Use AS collision theory, concentration, graph gradients, and balanced reaction equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to k as temperature rises for a typical reaction?
What you will learn
- Relate temperature to the rate constant
- Use activation energy to explain the increased successful-collision fraction
- Distinguish rate changes from concentration changes
Higher temperature increases the fraction of successful collisions
At higher temperature, particles have a greater range of kinetic energies and a larger fraction have energy at least equal to the activation energy. More collisions can therefore lead to reaction per unit time.
The rate constant k increases with temperature because the fraction of particles able to react successfully increases. Collision frequency may also increase, but the increased fraction above activation energy is the key explanation.
Rate can change even at unchanged concentration
If concentrations are held constant, a higher temperature changes k and therefore changes rate through the rate equation. This is distinct from increasing concentration, which changes the concentration term.
Do not claim that temperature changes the reaction order or that every collision becomes successful. The explanation is qualitative for this syllabus outcome.
Pause and explain
Why do more collisions lead to reaction at higher temperature?
Worked example
Explain why the rate of a reaction generally increases when temperature rises while concentrations stay fixed.
Show the worked solution
- State that concentrations are unchanged.
- Explain the higher fraction of particles with energy ≥ Ea.
- Connect successful collisions to k and rate.
Answer The rate constant increases because a larger fraction of particles have energy at least the activation energy, so more collisions are successful per unit time. With concentrations fixed, the increased k raises the rate.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A reaction mixture is warmed while concentrations stay constant. Explain the change in rate using k and activation energy.
Give me a hint
Use the larger fraction of particles with sufficient energy.
Compare my reasoning
- Temperature rises.
- More particles meet or exceed Ea.
- k and therefore rate increase at fixed concentrations.
A greater fraction of collisions have energy ≥ Ea, increasing successful reaction events. k increases, so the rate rises when concentrations remain fixed.
Look for these in your work
- Conditions distinguished
- Activation energy explained
- k-to-rate link stated
A student says warming a mixture increases rate because it increases [A]. Explain the error if the mixture was not diluted or concentrated.
Give me a hint
Temperature and concentration are separate terms in the rate equation.
Compare my reasoning
- The stated concentration is unchanged.
- Warming increases k.
- Explain the successful-collision fraction.
The concentration did not change. Temperature raises k by increasing the fraction of particles with energy at least Ea, so rate can increase at unchanged concentrations.
Look for these in your work
- Concentration held fixed
- k distinguished
- Energy explanation correct
Explain why the rate constant increases with temperature without claiming that every collision reacts.
Give me a hint
Describe a change in the energy distribution, not certainty for each collision.
Compare my reasoning
- The distribution includes more particles above Ea.
- Only those with suitable collision conditions can react.
- Therefore successful events per time rise.
At higher temperature a larger fraction of particles have energy ≥ Ea, so the frequency of successful collisions increases. Not every collision is successful.
Look for these in your work
- Energy distribution described
- Collision qualification included
- Rate-constant trend explained
Common mistakes
- Saying all particles gain the same energy
- Saying concentration rose when it was fixed
- Claiming every collision is successful
- Confusing rate constant with reaction order
What can you explain now?
What happens to k as temperature rises for a typical reaction?
Compare with the explanation
It increases
A greater fraction of particles can overcome activation energy.
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Make it stick
How does the energy distribution explain a temperature-driven increase in k and rate?
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